The invention relates to a cone for a display tube, which cone is made of cone glass according to the invention. The invention further relates to a method of preparing cone glass for a display tube by mixing a first portion of glass fragments having a known composition with a second portion of vitrifying materials.
Such a cone glass, method and the products obtained therefrom are known from U.S. Pat. No. 5,725,627 (PHN 14.670) in the name of the applicant. This known United States patent discloses a cone glass having a proportionally wide range of different components for the cone glass to be formed. Practice has proved that, with ever increasing stricter requirements being imposed on given properties of the cone glass, it is difficult or even impossible to use quantities of more than 5% by weight of recycled glass fragments of components of the used display tube as a starting material for new cone glass. It is currently common practice to use relatively small quantities of recycled glass for preparing new cone glass. The most important properties of the cone glass are thermal coefficient of expansion, viscosity, electrical resistance and X-ray absorption. Also when using variable quantities of recycled glass fragments, these properties should not vary too much because this may render the process of designing and further processing the cone glass impossible. Since it is very desirable for environmental reasons to use used display tubes as a starting material for new cone glass, further research has been done on compositions for cone glass on the possibility of using up to 40% by weight of recycled glass fragments, while searching for given small ranges of the composite components so that a range of compositions for cone glass is obtained within which the physical properties hardly vary.
After intensive research, a cone glass composition was found with which it is possible to use variable quantities (up to 40% by weight) of recycled glass fragments in the glass factory, where vitrifying components are added to the glass fragments so as to obtain a cone glass according to the invention in which the physical properties of the cone glass obtained are hardly changed.
The experiments which were performed yielded a cone glass having a composition as defined in claim 1, with which it is possible to obtain a cone glass having constant physical properties, independent of the quantity of glass fragments (up to 40% by weight).
It was surprisingly found that the range of compositions is large for given oxides such as SrO, BaO and PbO, namely larger than 2% by weight so that these oxides provide a wide tolerance in the use of recycled glass fragments, whereas the ranges are small, namely smaller than 1% by weight for other oxides such as Al2O3, Na2O, K2O, MgO and CaO, so that the presence of these oxides in the cone glass according to the invention should be adapted in a reasonably critical way by means of the other vitrifying materials which are added to the recycled glass fragments.
It is of great importance that the quantities by weight of the different oxides are adapted in a critical manner because the composition of display tube glass will strongly vary in the course of time and because the glass composition of the various display tubes is dependent on the supplier. It is therefore absolutely preferred to use display tubes made by the own manufacturer so that the glass composition is known. The government directives on recycling used glass, which will become even more stringent in the long term, necessitates many manufacturers to use a maximal number of discarded display tubes of television sets to prepare a cone glass from these tubes. This will ultimately also lead to a financial advantage for the display tube manufacturer.
The display tube actually comprises three parts, namely a screen, a cone and a neck. The neck constitutes only approximately 1% by weight of the total quantity of glass in the display tube. Proportionally, the cone glass contains little barium and strontium oxide but much lead oxide, while the glass from which the screen is made comprises barium oxide and strontium oxide but little or no lead oxide. The critical character of the most important-oxides in the composition of the cone glass according to the invention will hereinafter be elucidated with reference to the following comparative examples.
The most critical physical properties further stated in the examples are:
X-ray absorption (important for safety). A minimal value of 64.8 cmxe2x88x921 is required. A higher value is admitted, but a lower value is not.
Coefficient of expansion. This property is important for processing the cone glass to a final product, for the cone must fit on the screen. For this reason, the coefficient of expansion should be within narrow limits, namely 9.9xc2x10.1xc3x9710xe2x88x926Kxe2x88x921, at least in the temperature range between 25xc2x0 C. and 300xc2x0 C.
Viscosity. This property is important for melting and pressing. For the pressing process to be used, the working point is approximately 960xc2x0 C. A shift of maximally 5xc2x0 C. is admitted. The working point on the viscosity curve (logxcex7=4) is stated as an example.
Electrical resistance. This property is important for avoiding flashover at the high voltages in the tube. The logarithmic value of the resistance (measured in xcexa9xc2x7cm at 250xc2x0 C.) of the glass should therefore be minimally 9.6.
A cone glass according to the invention can be obtained by means of a method of preparing cone glass for a display tube, by mixing a first portion of glass fragments having a known composition with a second portion of vitrifying materials, in which a quantity of one or more X-ray-absorbing oxides in the second portion is chosen to be such that the cone glass has a minimal X-ray absorption of 64.8 cmxe2x88x921. The first portion of glass fragments may already comprise a quantity of the X-ray-absorbing oxides such as SrO, BaO, PbO and ZrO2. This quantity of X-ray-absorbing oxides must be replenished so that the cone glass has a minimal X-ray absorption of 64.8 cmxe2x88x921. Usually, only PbO is added in the second portion, but these may also be other X-ray-absorbing oxides.
Subsequently, the composition of the second portion is chosen to be such that at least one of the physical properties of electrical resistance, thermal coefficient of expansion and viscosity is in a desired range. Notably the quantity of Al2O3, Na2O, K2O, MgO and CaO in the second portion is varied for this purpose. In practice, usually only the quantities of Na2O and K2O are chosen to be such that the desired physical properties are obtained.
When the quantity of recycled fragments in the glass production is varied within very wide limits, from 0 to 40% by weight, then the contents of SrO, BaO, PbO and ZrO2 will accordingly vary within a wide range.
It has been found that, provided that the contents of Al2O3, MgO and CaO are held within narrow limits (max.xc2x10.3% by weight) (for which the basic glass materials of felspar and dolomite are responsible), the physical properties of the glass as mentioned above can be maintained constant by means of small corrections of the Na2O and K2O contents (hence with the basic materials Na carbonate and K carbonate).
The glass composition for a cone glass in which the percentage of recycled fragments may vary between 0 and 40% by weight and nevertheless has constant physical properties is as follows:
The rest, save for a refining agent (generally Sb2O3) and impurities, consists of SiO2.